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MaxHelixAngle
3rd Mar 2007, 10:28
Could someone please explain the difference between series and shunt wound generators and how this applies as the load on the system is increased.
Also I am a very confused as to how the voltage regulator works in the first place, as I understand it when the load is increased if nothing is done the system voltage will drop, so the voltage regulator supplies more current to the generator field which increases the magnetic flux and therefore the voltage. But as I understand it the output voltage of the generator is only affected by changing the rate of change of magnetic flux through the armature, so should only be affected by changes in RPM and magnetic field strength and if the RPM must be constant so the system Hertz can remain the same then why would the voltage change as system load increases??????
In short my question is for both a series and shunt wound generator what happens when the load increases so that voltage and cycles remains unchanged?
I realize this is a very large topic but if anyone could point me in the right direction it would be much appreciated.
Regards,
MHA

Lancelot37
3rd Mar 2007, 13:40
It's 54 years since I served an apprenticeship as an armature winder, so the memory is dull, especially after 4 G&Ts, two glasses of red and 4 whiskies, but you seem to be mixing ac and dc in your question. No shunt and series fields in an ac alternator, only in dc.

I'm sure that someone will come up with some answers, but I'll look back in tomorrow when the eyes can focus better. Hic.

(On holiday for 3 months in Mandurah, Australia)

kissmysquirrel
3rd Mar 2007, 13:52
There does seem to be a bit of confusion about which generator you are looking at. DC will have different windings ie shunt/series but the ac gen is pretty straightforward.

a google search will be very revealing.

http://micro.magnet.fsu.edu/electromag/java/generator/ac.html

for a basic view.

You seem to know what you are talking about but need to sort out whether its dc or ac you're interested in.

L Peacock
3rd Mar 2007, 14:55
Some mummblings

All traditional rotary generators are essentially AC., they have to be adapted to produce DC (split slip rings, electronic rectification etc.).
In a simple generator, you'd have fixed windings with a rotating permanent magnet. As you suggest, the only way you could vary the output voltage would be varying the shaft speed; this would render the frequency uncontrollable (A380??). The rotor therefore is designed as an electromagnet. The armature is rotated at whatever speed is required to maintain frequency (a function of the RPM (RPS) and number of pole pairs on the stator). You can see that if zero voltage if applied to the rotor, there would be no magnetic field and hence no voltage on the stator. Increase the voltage across and current through the rotor and you increase your stator voltage. All you need then is automatic feedback to create a closed loop system and controlled output voltage.

Lancelot37
4th Mar 2007, 02:07
Generators are DC. Slip rings are used to produce AC. The armature is only a permanent magnet on simple machines. There is a rotating magnetic field, the strength of which can be varied to alter the output on larger and more complex machines, such as on an aircraft.

Rotational speed does not vary to alter the output voltage. This is done by varying the magnetic field in the armature circuit. On an AC machine altering the rotational speed changes the frequency. (Hertz)

In modern machines there are many variations of the above and control systems are complex, but all follow the basic principal.

MaxHelixAngle
4th Mar 2007, 03:39
Thanks everyone for their help.

So as I now understand it: If the load is increased and nothing is done the voltage in the system will drop. In the voltage regulator a feedback circuit detects this drop and increases the current to the electromagnet thereby increasing the strength of the magnetic field and returning the voltage to a preset value. The RPM cannot be changed as this would affect the frequency.

So if output voltage is only governed by the RPM and magnetic field strength and RPM remains constant, it must therefore be the addition of load that is decreasing the output voltage making an increase in magnetic field strength necessary. Is this correct?

Kind regards,
MHA

P.S. Lancelot hope you enjoying the W.A. weather, does get a little boring after a while though!

Lancelot37
4th Mar 2007, 08:36
MaxHelixAngle, that sounds correct to me.

Yes, I must admit it's nice if we get a cloudy day with the temperature in the lower 20Cs. We are due for some 40Cs in the coming week.

Now there's an interesting one.

How much power loss is there from a jet engine when the temp is 45C compared to a UK departure temp. of 10C.

L Peacock
4th Mar 2007, 15:44
Lancelot

"Generators are DC." Nope. All roatating generators have to be essentially AC. Voltage can only be generated by rate of change of magnetic flux. Un-modified, the result will always be an AC waveform.

"The armature is only a permanent magnet on simple machines." Agree and I said the same in my post.

"Rotational speed does not vary to alter the output voltage." Yes it can on a simple machine again as I said in my post.

"This is done by varying the magnetic field in the armature circuit. On an AC machine altering the rotational speed changes the frequency. (Hertz)" Think I said that as well.

Sorry if I appear a little touchy but you seem to have disagreed with everything I stated for no particular reason.:ouch:

blackmail
4th Mar 2007, 18:59
hello maxhelixangle & others,

two important formulae for rotating dc machines :
1) E(voltage) = k(constant) x Magnetic flux( SI unit: weber) x omega( rotational speed in radians/second). this means that a rotating coil in a magnetic field produces a voltage potential(power source), which then can be connected to an electric circuit. in aviation this is a dc generator, connected to so called "bus bars"(+) or metal strips, where different users( pumps, light bulbs etc. ...) are hooked on. as the negative(-) or earth connection, the aircraft airframe is used, saving 50% of the wiring needed.
2) M(moment in meter newton) = k(constant) x Magnetic flux x Ia(amperage or amps). this means that, if ampères are send through a wire, which is in a magnetic field, a force or a moment(for rotational devices) is created. to determine the direction of the force mr. Fleming rule is used with your right hand: palm of right hand on top of magnetic flux, thumb in direction of electric current(amps), then other straight fingers indicate force direction. there is also another rule called the rule of the three fingers to find same direction.

but now back to our generators.
concerning the choice between a shunt or a series wired dc generator, in aviation, only shunt dc generators are used for following reasons :
1) for the voltage regulator( which influences the magnetic flux), a series voltage regulator would be very difficult & costly to manufacture, mechanically as well as electrically. a shunt voltage regulator, on the otherhand, would be very easy & cheap to produce.
2) concerning reverse currents(short circuits). for a series wired dc generator, the magnetic flux would be reduced dramatically & the moment being constant, the amperage would increase catastrophically( see second formula above), resulting in a burned out generator, as where, for a shunt dc generator, a reverse current would just cause the generator to swap function & to run as a dc motor, which is of course much better than to burst in flames.

hope this helps, but i need a cup of coffee now,
kind regards,
bm